THEREIN

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

[0001] This application claims the benefit of priority under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application 61/623,595, filed April 13, 2012, entitled "METHOD OF MAKING DENTAL PROSTHESIS AND DUCTILE ALLOYS FOR USE THEREIN," the disclosure of which is hereby incorporated herein by reference in its entirety.

BACKGROUND

Field

[0002] Embodiments of the present disclosure relate generally to the manufacturing of dental prostheses using a selective laser melting process and, more specifically, to ductile cobalt-ruthenium-chromium alloys for use in the process.

Description of the Related Art

[0003] The traditional materials used for the fabrication of dental prosthetic devices have been gold and palladium based alloys. Over the last fifteen years the increasing prices of gold and palladium have prompted a search for lower cost substitute materials. Such alloys are described by Cascone in U.S. Patent No. 7,794,652, hereby incorporated by reference in its entirety.

SUMMARY

[0004] Briefly stated, some embodiments of the present disclosure are directed to a method for making a dental prosthesis which can comprise the steps of (a) providing a pre-alloyed fine powder comprising in % by weight: 30-40% Co, 25-40% Ru, and 20-40% Cr; and (b) forming the dental prosthesis by selective laser melting the pre- alloyed powder in a mold of a selected shape. The dental prosthesis metal alloy pre-form can be then surface coated (veneering) with dental porcelain. The coefficient of thermal expansion of the metal alloy can closely match that of the dental porcelain so as to prevent cracking during high temperature processing of the porcelain. Alternatively, in some embodiments, the method can comprise the steps of (a) providing a molten bath of a ductile alloy comprising in % by weight: 30-40% Co, 25-40% Ru, and 20-40% Cr; casting the molten alloy into a mold to form a near-net shape pre-form or blank of a dental prosthesis; machining, as by grinding, the pre-form or blank to a selected shape; and coating the machined shape with a dental porcelain.

[0005] In some embodiments, a method of making a dental prosthesis can comprise providing a metal alloy powder comprising, in % by weight, about 30 to about 40% Co, about 25 to about 40% selected from the group consisting of Au, Pt group, and combinations thereof, about 20 to about 40% Cr, and 0 to about 0.1% Ni, wherein the metal alloy powder can comprise at least 20% Ru. The method can further comprise melting the alloy powder using selective laser melting to form a metal alloy pre-form. In some embodiments, providing the metal alloy powder can comprise providing a first powder layer of the metal alloy powder, and melting the alloy powder using selective laser melting can comprise directing at least one laser to melt the first powder layer on a first specified x-y plane to form a layer having a given thickness, providing a second powder layer of the metal alloy powder, directing the at least one laser to melt the second powder layer on a second specified x-y plane to form a second layer having a second thickness, wherein the second layer is formed so that it is fused with the first specified x- y plane, and repeating the steps of adding a powder layer and directing the at least one laser until a metal alloy pre-form is formed. In some embodiments, the metal alloy powder can comprise about 25 to about 40% Ru.

[0006] In some embodiments, the method can further comprise applying a veneer of a dental porcelain to the metal alloy pre-form. A coefficient of thermal expansion of the metal alloy pre-form can be compatible with that of the dental porcelain to prevent cracking of the porcelain. In some embodiments, the metal alloy powder can comprise about 40% Co, about 30% Ru, and about 30% Cr and the metal alloy pre-form can have a coefficient of thermal expansion of about 12 (10 ^-6 /K at 600°C). In some embodiments, the metal alloy power can comprise about 25 to about 35% Ru, about 25 to about 30% Cr, and a balance of Co. In some embodiments, the metal alloy powder can comprise about 35% Co, about 35% Ru, and about 30% Cr.

[0007] In some embodiments, the dental prosthesis can be non-magnetic. The metal alloy pre-form can have a coefficient of thermal expansion of about 14 (10 ^-6 /K at 600°C). In some embodiments, the dental prosthesis can be substantially free of cracks or other defects. In some embodiments, the metal alloy powder can be substantially spherical. In some embodiments, the metal alloy pre-form can have a relative magnetic permeability of 1.00100. Additionally, a dental prosthesis can be formed from the above described methods.

[0008] In some embodiments, a dental prosthesis can comprise a metal alloy pre-form formed from an alloy powder through selective laser melting, wherein the alloy powder can comprise, in % by weight, about 30 to about 40% Co, about 25 to about 40% selected from the group consisting of Au, Pt group, and combinations thereof, about 20 to about 40% Cr, and 0 to about 0.1% Ni, wherein the metal alloy powder can comprise at least about 20% Ru. The dental prosthesis can also comprise a dental porcelain veneer coating the metal alloy. In some embodiments, the metal alloy powder can comprise about 25 to about 40% Ru.

[0009] In some embodiments, a coefficient of thermal expansion of the metal alloy pre-form can be compatible with that of the dental porcelain to prevent cracking of the porcelain. In some embodiments, the metal alloy powder can comprise about 40% Co, about 30% Ru, and about 30% Cr and the metal alloy pre-form can have a coefficient of thermal expansion of about 12 (10 ^-6 /K at 600°C). In some embodiments, the dental prosthesis can be non-magnetic.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Fig. 1 shows a dental coping or pre-form of an embodiment of the present disclosure; and

[0011] Fig. 2 shows a finished dental prosthesis or crown of an embodiment of the disclosure after a porcelain coating is applied to the coping of Fig. 1.

DETAILED DESCRIPTION

[0012] Embodiments of the present disclosure provide ductile alloys for use in medical products, for example dental prostheses, having a determinate composition, and methods of manufacturing them. In particular, the ductile alloys can have a coefficient of thermal expansion that is similar to porcelain materials that can overlay the alloy in dental prostheses. Moreover, the ductile alloys can be manufactured through the use of selective laser melting (SLM). [00131 The terms "approximately", "about", and "substantially" as used herein represent an amount close to the stated amount that still performs a desired function or achieves a desired result. For example, the terms "approximately", "about", and "substantially" may refer to an amount mat is within less than 10% of, within less than 5% of, within less than 1% of, within less than 0.1% of, and within less than 0.01% of the stated amount.

[0014] Dental prostheses can be made of multiple layers, for example, a metallic core can be formed and a porcelain veneer can overlay the metallic core. However, the porcelain overlay can be substantially more brittle man the underlying metallic core. Because of this, if the metal and porcelain are heated or cooled together, the metal can expand or contract at a higher rate than the porcelain, leading to high stresses on the porcelain. The high stresses can eventually result in failure of the porcelain through, for example, cracking. One approach to address the absorption of the stresses is to utilize alloys with similar coefficients of thermal expansion as that of the porcelain. Some ductile alloys are described by German Patent No. 1104195 ("Obrowski"), hereby incorporated by reference in its entirety. While not described in the patent itself, some of the ranges described by Obrowski (e.g. 29-45 wt% cobalt, 20-50 wt% ruthenium, and 20-40 wt% chromium) can be broad enough to accommodate altering the thermal expansion of the alloy in order to be compatible with different dental porcelains. If the coefficients of thermal expansion of the alloy and the dental porcelain closely match, this can prevent cracking of the porcelain during processing, which can take place at about 900°C. Currently, dental porcelains have a coefficient of thermal expansion of between about 12-14 (10 ^-6 /K at 600°C).

[0015] In some embodiments, the alloy used in the present disclosure for the manufacture of dental prostheses can be in % by weight: 30-40% (or about 30% to about 40%) Co, 25-40% (or about 25% to about 40%) Ru, 20-40% (or about 20% to about 40%) Cr, and 0-0.1% (or 0 to about 0.1%) Ni, preferably about 25-35% (or about 25% to about 35%) Ru, 25-30% (or about 25% to about 30%) Cr, and a balance of Co, more preferably 35% (or about 35%) Co, 30% (or about 30%) Cr, and 35% (or about 35%) Ru. The alloy in some embodiments can contain a minimum of 25% (or about 25%) Au and platinum group elements, including Pt, Rh, Os, Pd, Ir, and Ru. In some embodiments, the alloy can contain a combination of Au and platinum group elements. In some embodiments, the 25% Au and platinum group elements can be at least about 20% Ru, wherein the 20% Ru is the % of the alloy. By employing ruthenium in the alloy composition, the intrinsic cost of the alloys can be lowered while still maintaining the ADA Classification of Noble (alloys that contain at least 25 weight percent gold and/or platinum group elements).

[0016] The alloy can be most preferably free of Ni and the finished part can exhibit weak ferromagnetic properties, that is, the part is only weakly attracted to a magnet. In some embodiments, such as those having higher percentages of Cr, such as 25% (or about 25%) or above, the alloy may not be ferromagnetic at all, and may in fact be non-magnetic. This differs from Obrowski, which mainly discloses particular compositions using low chromium content, which almost always leads to a ferromagnetic material. Further, the alloy can be corrosion resistant, and can have a relative magnetic permeability of 1.00100.

[0017] In some embodiments, iron can be alloyed in to the material while still maintaining ductility. In some embodiments, the coefficient of thermal expansion of the disclosed alloys can be 12, 13, or 14 (or about 12, 13, or 14) (10 ^-6 /K at 600°C).

Selective Laser Melting

[0018] The dental laboratory industry has been exploring more efficient methodologies in fabricating dental prosthesis. One technology that holds particular promise is selective laser melting (SLM). While, as described above, the disclosed alloy composition can be useful in achieving similar thermal coefficients of friction as porcelain, the alloys can also be used during the SLM process for manufacturing due to, at least, the ductility of the alloys.

[0019] In some embodiments, selective laser melting can be used to construct a dental prosthesis from a metallic alloy, such as the alloy described above. SLM can be used to construct a three-dimensional object from a given electronic file that can define the boundaries, shape, interior structure, porosity, as well as other properties for a given object from a powder.

[0020] In some embodiments, a three-dimensional object, such as a coping for a single tooth or a framework for a bridge, can be created in, for example, a computer program, to create an electronic file. In some embodiments, 3D CAD data can be used as a digital information source to create the three-dimensional object. The object can be stored in a file format, such as an industry standard STL file for most layer-based 3D printing or stereo lithography and the file can be sent to a machine which performs SLM. This file can then be loaded into a file preparation software package that assigns parameters, values and physical supports that allow the file to be interpreted and built by different types of additive manufacturing machines. Once the electronic file is stored within a SLM machine, the file can be converted, or sliced, into a series of layers of a specified thickness extending in the vertical axis along a specified x-y plane, creating a 2D image of each layer. These layers can be stacked one on top of the other to form the final three-dimensional object. In some embodiments, the layers can be 20 to 100 (or approximately 20 to 100) micrometers thick.

[0021] The material to be used in the SLM can be collected and atomized to form a fine powder. In some embodiments, powder of the alloy described above can be used to form a dental prosthesis. In some embodiments, the powder is spherical in shape; however other shaped powders, such as generally irregularly shaped powders, can be used.

[0022] A first layer of powder can evenly spread upon a platform, such as a substrate plate, in which the three-dimensional object can be built In some embodiments, the platform can be a generally flat surface and made of a metal. Additionally, the platform can be configured to be raised and lowered in the vertical (Z) axis, such as by being fastened to an indexing table that can move in the Z axis. The powder can be spread in an even layer by, for example, a machine or mechanical process. Once the powder is spread, energy, such as a high powered laser beam, such as an ytterbium fiber laser, can be directed along the two dimensional layers described in the electronic file to melt a layer of the powder into the shape specified in the file. The laser energy can be intense enough to permit full melting (welding) of any particles to form a solid piece. A second powder layer can be provided, and this process can be repeated layer after layer in the vertical axis until the entire three-dimensional object has been created. In some embodiments, the lasers can be raised or the platform can be lowered so that the vertical layers are formed. In some embodiments, SLM can be carried out in a tightly controlled protective environment, such as with nitrogen or argon with an oxygen level below 500 ppm.

[0023] During the laser melting process, the powder can absorb the laser light, and therefore melt. While melting a given layer, the previous layer can also be partially melted, and therefore the two layers of melted powder can be joined, fused, or welded together. As the laser moves away from the two dimensional point, the melted powder solidifies, thereby forming a portion of the three-dimensional object. In some embodiments, once each layer has been distributed, each 2D slice of the object geometry can be fused by selectively applying the laser energy to the powder surface, such as by directing the focused laser beam using two high frequency scanning mirrors in the X and Y axes. SLM can be programmed to form an object with a specified porosity, or with a specified structure within the three-dimensional object.

[0024] It is known to manufacture dental prostheses using direct metal laser sintering by fusing cobalt chromium alloy powder using a laser, see: www.eos.info/en/applications/. hereby incorporated by reference in its entirety. However, the SLM process requires the use of fine alloy power. When alloys described by Cascone in U.S. Patent No. 7,794,652, hereby incorporated by reference in its entirety, are atomized and used in the SLM machine, the parts fracture. The cause of the fractures is suspected to be large thermal stresses that arise during the rapid melting by the laser and solidification of the alloy. As the powder used in SLM is rapidly heated and cooled during SLM, significant stresses can be experienced by the part as it is being formed. This residual heating and cooling stress may be large enough to cause fracturing of the part.

[0025] The alloys described above can be used with selective laser melting without fracturing, as the alloys exhibit ductile behavior. This ductile behavior may be especially suited for the SLM process, as ductile materials are able to better withstand the rapid heating and cooling. Ductile behavior can include, for example, high elongation or low yield strength at high temperatures. If a material were to have one, or both, of these ductile behaviors, they may be suited for use with SLM, as they may be more capable of handling the high amounts of thermal stress placed on the material during the SLM material. By being able to absorb and or resolve the thermal stress, the material can proceed through the SLM process without damage, such as fractures.

Examples

[0026] A first sample of an embodiment of the alloy of the disclosure was prepared by melting 35 wt% Co, 35 wt% Ru, and 30 wt% Cr in an induction heated crucible in an argon atmosphere. The molten alloy was then atomized to form a pre- alloyed powder. The powder was sized by screening - 45 um + 10 um. The screened fine, pre-alloyed powder was then introduced to a selective laser melting (SLM) machine to laser melt the powder and fill a mold which has been previously made to the desired shape of the dental coping (one tooth) or bridge (more than one tooth). The finished coping is shown in Fig. 1 wherein the SLM alloy is in a solidified condition. The first sample of the alloy after melting and solidification had a coefficient of thermal expansion of about 13 (10 ^-6 /K at 600°C). The coping of Fig. 1 is then sent to a dental lab for coating (veneering) with a dental porcelain as shown in Fig. 2 as a finished dental prosthesis in the form of a crown.

[0027] it should be mentioned that the creation of a mold for a dental coping is, in itself, well known in the art and need not be explained in detail herein.

[0028] In a further embodiment of the present disclosure, the molten alloy of the above-described composition may be cast into a mold to form a near net shape of the coping or cast as a block or blank of metal alloy. The solidified shape or blank is then machined by grinding, for example, into the finalized prosthesis shape.

[0029] As mentioned above, the alloy of the disclosure preferably contains less than 0.1 wt% (or less than about 0.1 wt%) Ni and, more preferably, contains no Ni. The finished part does not exhibit strong ferromagnetic properties, which is desirable in a dental prosthesis.

[0030] A second sample of an embodiment of the alloy of the present disclosure was prepared, having a composition containing: 40 wt% Co, 30 wt% Ru, and 30 wt% Cr. The second sample was processed in the same manner as described above with respect to the first sample alloy, including the selective laser melting. The solidified metal alloy of the second sample had a coefficient of thermal expansion of about 12 (10- ⁶ /K at 600°C). The second sample thus provided a substantially perfect match with a dental porcelain veneer having a coefficient of thermal expansion of 12 (10 ^~ /K at 600°C). As mentioned above, this close match in the thermal expansion of the metal alloy and the porcelain veneer provides a crack-free dental prosthesis.

[0031] While specific embodiments of the disclosure have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. The presently preferred embodiments described herein are meant to be illustrative only and not limiting as to the scope of the disclosure which is to be given the full breadth of the appended claims and any and all equivalents thereof.